In present satellite transmission systems, data transmitted through the satellite link is partitioned into blocks. The partitioned data is (among other things) scrambled to produce an output signal having a relatively flat spectrum, thus optimizing the use of the bandwidth. Synchronization (sync) words, which may have one of two values (described in more detail below), are then added to the beginning of each block, and the resulting stream of data words is Reed-Solomon encoded to permit detection and correction of data words corrupted by noise during the transmission process. Circuits in the receiver recognize and process the sync word in each block to synchronize the receiver circuitry to the blocks in the received signal. Then the data in each block is (among other things) Reed-Solomon decoded and descrambled to extract the transmitted data.
The descrambling circuitry in the receiver operates by exclusive-ORing a descrambling sequence with the received scrambled data to produce unscrambled data for subsequent processing. In order to operate properly, the descrambling sequence must periodically be reset. In an embodiment currently in use in European satellite digital TV systems, the descrambling sequence must be reset every eight blocks. A sync word having a first value is normally included at the beginning of each block, but an inverted sync word (i.e. one having a value which is the logical inverse of the value of the normal sync word) is included at the beginning of every eighth block to indicate that the descrambling sequence must be reset. Current receiver circuitry detects the inverted sync word at the beginning of a block and resets the descrambling sequence.
A problem with such a system occurs when an inverted sync word and other data in its associated block become so corrupted that the Reed-Solomon decoder cannot correct it. In this case, no inverted sync word is detected by the receiver circuitry, and the descrambler is not reset. Consequently, all following blocks until the next successful reception of an inverted sync word are incorrectly descrambled, even though they may have been properly corrected by the Reed Solomon decoder. The descrambling sequence cannot be reset until some integer multiple of eight blocks have passed, i.e. at least eight blocks.